Peel and coarsely chop potatoes.,Trim leeks, slice in half longwise and separate under running water to remove all sand and dirt, then coarsely chop.,Clean and quarter mushrooms.,______,Add salt, about 2 teaspoons, and bring to boil.,Simmer until potatoes are tender, about 20 minutes.,Ladle 1/2 of cooked potato mixture into food processor and add half of the spinach.,Process until smooth and thick.,Repeat with rest of potatoes and spinach.,Add about 1 Tablespoon butter buds for flavor and thin with skim milk if needed.,Serve hot.